The role of ultraviolet light (UVL) and sunlight in the etiology of certain types of skin cancer seems well established. The mechanism(s) associated with these processes are not understood. It has been suggested that the carcinogenic activity of light is dependent upon the photochemical conversion of sterols of carcinogenic substances. The formation of a sterol- derived photoproduct with carcinogenic properties has now been demonstrated in UV-irradiated human and hairless mouse skin. The role of this compound cholesterol alpha-oxide, in the etiology of UV-carcinogenesis is under investigation. Levels of the compound, in skin of animals receiving chronic, suberythemic levels of UVL, increase 2 to 8 fold at 10 weeks after which squamous cell carcinomas appear. These data suggest a causal relationship for this compound or metabolites thereof. As cholesterol alpha-oxide is formed as a result of photo-oxidation, prevention of its formation by antioxidants should be accompanied by prolongation of tumor induction times or lower percentage of induced tumors. If the compound is, indeed, involved in the disease development, then future investigations into the mechanism of its formation, its metabolism by the target tissue, and its subcellular localization will be undertaken.